1. Field of the Invention
The invention relates to the modulation and filtering of digital signals, in particular of signals which are usually called binary data signals.
Data signals, which consist of a succession of signal elements of uniform duration whose amplitude represents a binary "0" or binary "1", are not simply suitable for transmission via, for example, a telephone channel. It has therefore become normal practice to use so-called "modems" in which the data signals before transmission modulate a carrier wave and in which after reception the modulated signal is demodulated.
Any of the well-known modulation techniques such as FSK, PSK, AM-DSB, AM-SSB, AM-VSB can be used for the realization of a frequency spectrum suitable for transmission via a telephone channel.
The invention results from investigations in the field of a 2400 Baud AM-VSB modem, but is not restricted to this field alone, as the same principles can be used for other data speeds and for other linear modulation methods such as AM-DSB, AM-SSB, FSK (modulation-index 1/2 ) and PSK. Although in what follows hereafter, reference is made to a 2400 bit/sec AM-VSB modem and its extension to a 4800 bit/sec AM-VSB modem this should not be interpreted as a restriction of the possibilities of application of the principles according to the invention.
2. Description of the State of the Art.
According to a frequently used method in conventional AM-VSB modems the data signals are first filtered in a low-pass filter. The filtered signals modulate a carrier wave in amplitude and, of the sidebands of the modulated signal, one of the sidebands plus a part of the other sideband is selected by a vestigial sideband filter whose output signal is transmitted.
The low-pass filter causes a limitation of the frequency spectrum of the data signals at the side of the high signal frequencies. At the, with regard to the highest data signal frequency, relatively low carrier frequency of data modems for telephone channels, the low-pass filter also prevents the phenomenon of foldover which occurs when high signal frequencies are superimposed, after modulation on a carrier wave, in the area which is occupied by the lower signal frequencies after modulation.
The low-pass filter is usually called the pre-modulation filter and the vestigial sideband filter the postmodulation filter. Modulation takes place between these two filters.
The output signal of the pre-modulation filter is an analog signal, even when the input signal is a binary signal, so that an analog modulator must be used.
It has been proposed to replace the analog modulator by a simple logic circuit and to interchange the order of the steps of pre-modulation filtering and modulation in order to profit from the binary form of the input signal. It has been proved that it is only possible to interchange these steps when the carrier frequency amounts to an integral multiple of one half the clock frequency of the digital input signal. Only in this case the resulting foldover distortion can be compensated by a linear network.
For practical purposes this is of limited importance, as may appear from a provisional proposal by the CCITT for a 4800 bit/sec AM-VSB modem, that this modem must have a carrier frequency of 2100 Hz, which is not in the said proportion to the clock frequency (in this case 2400 Hz at four-level coding).
Interchange of the steps of pre-modulation, filtering and modulation, which is only permissible in a limited number of cases, results in that only one filter is still needed, namely a post-modulation filter. This filter can be designed in such a way that the foldover distortion is compensated by it.
By realizing the post-modulation filter as a binary transversal filter, a data modem can be realized in the proposed manner from digital integrated circuits.
According to another line of development and with the purpose of mitigating the requriements the post-modulation filter must satisfy it has been suggested to realize the pre-modulation filter as a band filter. This proposal is based on the fact that by the use of short pulses for the representation of the binary information the signal energies at the higher frequencies, which also fully represent the binary information, are amplified. By selecting these higher-frequency signal components, a signal is obtained such that after modulation the sidebands thus created can be separated in a simple manner.